The invention relates to a water-in-oil macroemulsion for use in combination with a solid oxidizer in preparing explosive compositions. When combined with a solid oxidizer, the macroemulsion and oxidizer form an explosive composition having a density greater than that achievable by simply using fuel oil by itself with the solid oxidizer.
Mixtures of ammonium nitrate (AN) and diesel fuel oil (FO) have been used for many years in the explosives industry, and the mixture has come to be known as ANFO. Typically, ammonium nitrate in prill form is mixed with diesel fuel oil in the ratio of about 94 to 6 AN:FO. ANFO is inexpensive and is widely used in various kinds of blasting, but its relatively low bulk density (about 0.8 g/cc) limits the amount of useful energy that can be obtained per borehole. A higher density product would provide more energy per borehole. ANFO also becomes desensitized by water, thereby precluding its use in water-filled boreholes.
Various attempts have been made to increase the density or bulk strength of ANFO, and thereby provide more energy per volume. Some examples of these attempts include the use of high density additive fuels (e.g., ferrophosphorous), crushing the ammonium nitrate, and using thickened water-based AN slurries. These attempts have met with limited success. The use of high density fuels require special equipment for addition of the fuels to the prills which increases the cost of the explosive. Similarly, special equipment and extra personnel are required for partially crushing the prills which also leads to increased costs. Slurries have the problem of lacking sensitivity and require the addition of sensitizing agents as well as the use of additional equipment.
U.S. Pat. No. 3,764,421 describes one such attempt to solve the density problem of ANFO that includes the addition of water in controlled amounts to a prilled AN-fuel oil mixture. The mixture is aged for a period of time (typically 10 to 14 days) and then stirred to break down the AN into finely divided solids. This approach achieves essentially the same result as partially crushing the prills but uses aging instead of special equipment to accomplish the result. Extra time and equipment are required to gain an increase in density. A need still exists, therefore, for a method and formulation to increase the density of a solid oxidizer-based explosive composition over that obtainable with ANFO without the use of special equipment or a lengthy aging process. It would be highly desirable to have a solid oxidizer-based explosive that has a greater density than ANFO but could be mixed and loaded using the equipment presently available and used in mixing and loading ANFO. The present invention meets this need by providing a water-in-oil macroemulsion to be used with a solid oxidizer such as ammonium nitrate prills to provide a high density explosive.
The explosives art has used water-in-oil emulsions in formulating various types of explosive compositions. Almost always, the emulsion consists of an aqueous solution of oxidizing salts as the discontinuous phase, diesel fuel oil as the continuous phase, and an emulsifier. U.S. Pat. No. 3,447,978 to Bluhm discloses a water-in-oil emulsion explosive comprising a solution of oxidizing salts as the discontinuous aqueous phase, a fuel, an emulsifier, and occluded gas to lower the density of the emulsion and increase the sensitivity. Without the occluded gas, the emulsion is not detonable in useful diameters. Later patents, such as U.S. Pat. No. 3,765,964 disclose emulsions that include sensitizers such as strontium in addition to occluded gas to increase the sensitivity of the emulsion.
Numerous other patents also describe explosive emulsions that incorporate oxidizing agents in the aqueous phase of the emulsion. U.S. Pat. No. 3,161,551 to Egly, et al. discloses a water-in-oil emulsion containing 50-70% by weight of ammonium nitrate, 15-35% water, 5-20% of an organic sensitizer and a small amount of emulsifiers. Egly teaches to combine this AN-based emulsion with particulate ammonium nitrate so as to fill all the spaces between the particles. U.S. Pat. No. 3,356,547 to Berthmann, et al. describes an emulsion containing nitroglycerine that is used in combination with solid AN particles.
Clay in U.S. Pat. No. 4,111,727 discloses an explosive composition formed by mixing 10-40% of a water-in-oil emulsion containing an oxidizer salt dissolved in the aqueous phase with 60-90% of solid oxidizer such as ammonium nitrate prills. The emulsion and oxidizer are mixed such that sufficient air is left in the interstitial spaces of the solid oxidizer to render the mixture detonable. The emulsion does not need to contain occluded gas.
Clay in U.S. Pat. No. 4,181,546 discloses a waterproof explosive comprising 40-60% by weight of a solid, particulate oxidizer salt and 60-40% of a water-in-oil emulsion containing an oxidizer salt dissolved in the aqueous phase The emulsion also contains a density controlled sensitizer such as hollow glass beads, polystyrene beads, microballoons or the equivalent. The components are thoroughly mixed together to substantially eliminate the voids between the solids.
In a later patent, U.S. Pat. No. 4,294,633, Clay discloses a blasting composition comprising 60-90% by weight of solid particulate oxidizer salt and 10-40% of a liquid slurry partially filling the interstices and voids between the solid particles. The slurry is a substantially saturated and thickened solution of strong oxidizer salt and does not appreciably dissolve or soften the granules.
A disadvantage of water-in-oil emulsions in which the aqueous phase contains dissolved oxidizer salts is that the emulsions are highly viscous compared to diesel fuel oil and require special equipment and handling. Also, such emulsions are relatively unstable and will separate or "break" into different phases with time or with temperature cycling. When emulsions such as those described in the Clay, U.S. Pat. Nos. 4,181,546 and 4,111,727 are used, they are generally stored separately until mixed with the solid oxidizer particles. In order to prevent phase separation in cold climates, it is usually necessary to heat the emulsion continuously from the time of production until the time of use to prevent the oxidizer salt from crystallizing in the aqueous phase. These same disadvantages are characteristic of almost all of the emulsions currently used in the explosive industry as almost all of them exhibit limited stability over time and sensitivity to low temperatures.
U.S. Pat. No. 4,555,278 to Cescon, et al. describes a relatively stable blend of nitrate particles and a water-in-oil emulsion comprising an aqueous solution of oxidizing salts, a liquid fuel, and an emulsifying agent. The blend also contains a sensitizing amount of dispersed gas bubbles or voids. The patent is directed towards forming a composition that minimizes the transfer of water from the aqueous phase across the continuous phase to the nitrate particles.
Another example of a water-in-oil emulsion explosive is disclosed in U.S. Pat. No. 4,357,184 to Binet. The disclosed emulsions comprise an aqueous solution of at least one oxygen supplying salt as a discontinuous phase, a liquefiable carbonaceous fuel as a continuous phase, a sensitizing component and a blend of emulsifying agents. Binet describes these emulsions as "ultra-stable colloidal dispersions" and uses the term microemulsion. As used by Binet, the term microemulsion describes a liquid-liquid foam of very small cell size ranging from about 1 micron to about 15 microns. In the emulsion art, however, the term microemulsion means something different than that used by Binet. What Binet termed a microemulsion is more properly termed a small cell macroemulsion.
Australian patent No. 281,537 to Coxon describes an oil-in-water emulsion containing an anionic surface active agent that is mixed with AN prills. Coxon prefers AN prills that are processed so as to have a high oil absorption. Coxon only discloses the use of an oil-in-water emulsion formed by using anionic surface active agents in the amount of 2.5% to about 7.5% by weight of the emulsion. Coxon teaches that nonionic surface active agents produce caking of the ANFO mixture and the product loses its free-flowing properties. Coxon states that cationic surface active agents are not generally used because some of them liberate ammonia which affects the ammonium nitrate. The HLB values for the surfactants disclosed in Coxon are relatively high and range from about 11 to as high as 40. Coxon discovered that he could improve the sensitivity of ANFO by adding a small amount of water and distributing it with the oil in the form of an emulsion over the ammonium nitrate prills. In this manner, Coxon desired to achieve greater intimacy between the oil and the AN thereby achieving greater sensitivity. Coxon used an oil-in-water emulsion with anionic surfactants to accomplish these results.
The anionic surfactants disclosed in Coxon produce almost exclusively oil-in-water emulsions. These emulsions are not as suited for use in combination with a solid oxidizer as a replacement for ANFO as are the water-in-oil emulsions of the present invention. The oil-in-water emulsions disclosed in Coxon are useful only over a relatively narrow range of temperatures. Since water is the continuous phase, Coxon's emulsions have a separation temperature and freezing point of about 0.degree. C. In contrast, the water-in-oil emulsions of the present invention exhibit a much lower separation temperature, as low as -30.degree. C. and lower, and therefore, have a much broader useful range. As mentioned, one of the problems of previously known emulsion-type explosive systems is the need to keep them heated to prevent separation of the emulsion in cold climates. Coxon's emulsions would also require constant heating to prevent freezing and the separation of the emulsion at temperatures below the freezing point of water. Another problem with Coxon's emulsions is that they could not be used with the same equipment and piping systems as currently used for ANFO. This equipment is almost exclusively made of iron or steel which would be subject to severe corrosion if used with Coxon's emulsions in which water is the external phase. These corrosion problems are minimized with the water-in-oil emulsions of the present invention.
The present invention provides an explosive composition that can be used as a replacement for ANFO, or in combination with ANFO, while using the same equipment and essentially the same processes as are currently used for ANFO and providing an explosive composition with a greater density and bulk strength than ANFO. These features result in a very desirable explosive composition.